This invention relates generally to adaptive equalizers. More particularly, this invention relates to a pulse width controlled adaptive equalizer. The pulse width controlled adaptive equalizer of the invention is not limited to, but finds particular use in the receiver section of a coaxial cable line interface device which recovers and transmits DS3 and STS-1 telecommunication signals.
DS3 and STS-1 telecommunication signals are governed by standards which require that the signals be transmitted at certain amplitudes or power. Typically, the standards suggest that the signal be transmitted at a particular power .+-.3.5 dB. Effectively, then, a 7 dB range is possible. Additionally, in a "bridging mode", an additional attenuation of 20 dB is typical. Further, where the DS3 or STS-1 signal is being transmitted in a central office or the like by coaxial cables of up to four hundred fifty feet in length, an additional attenuation of up to 6 dB may be experienced. While the bridging mode attenuation is not frequency dependent, and the power range of the DS3 or STS-1 signals has only a small frequency dependence, the attenuation due to the coaxial cable is known to be strongly frequency dependent according to a root-f (.sqroot.f) function. Thus, in correcting for gain, it is important to know the extent to which the low amplitude of the received signal is due to the original power of the signal sent and the extent to which it is due to attenuation in the coaxial cable. Standard equalizers which use the signal amplitude for equalization ignore the distinction of the types of attenuation, and either must accommodate for the frequency dependence of the attenuation in other manners, or suffer from poor results.
An addition to providing an amplitude independent means of equalization, ideally, a pulse width controlled output set to one-half the period of the system frequency is generated. This equalized pulse width controlled output can then be used as an input to a pulse-width sensitive phase detector which is typically used in a phase locked loop clock recovery circuit for telecommunication signals. Since the prior art amplitude sensing adaptive equalizers do not control the output pulse widths of their recovered data signals, the jitter performance of clock recovery circuits used with these equalizers is degraded by non-ideal centering of the recovered clock with respect to the received data pulses.